1. Field of the Invention
The present invention relates to an image search technique for searching an image database storing images, for desired images.
2. Description of the Related Art
Known methods of searching for an image can be roughly classified into two types.
According to a first search method, images are provided with keywords that reflect the contents of the images. Then, during a search, an image provided with a keyword equivalent to one input by a user is extracted from an image database and presented to the user.
A problem with this search method is that the operation of providing each image with an appropriate keyword is not easy. Furthermore, if the user is different from the person who has provided the keywords, the reference keyword may not match the corresponding keyword used in the image database even though the two keywords are conceptually the same. Thus, this method may fail to search for appropriate images.
According to a second search method, a search is carried out utilizing attribute values that quantify the physical characteristics of images such as their colors, shapes, or textures. The attribute values of a reference image are compared with those of a searched image to extract a similar image from the image database. This image is then presented to the user as a search result.
With this method, since the attribute values extracted on the basis of a predetermined algorithm are not necessarily the same as those of an equivalent image human beings expect to obtain, the similarity between the searched image and the reference image may often be low in terms of human senses. Consequently, it has been pointed out that this method has a low detection accuracy.
To avoid these problems, the following technique has been proposed. For a set of images in a database which are provided with the same keyword, characteristic amount vectors and importance levels are determined. Then, the keyword is converted into an attributed value. The database is then searched for an image on the basis of this attribute value (Jpn. Pat. Appln. KOKAI Publication No. 2002-140332).
However, with this search method, images must be provided with keywords. Thus, much labor is required to provide the keywords. Furthermore, the distribution of the characteristic amount vectors of images with the same keyword is not ensured to be sufficiently localized on a characteristic space. Consequently, similar images cannot always be retrieved accurately.
As an alternative, the following technique has been proposed. A search is carried out using keywords assigned to images. Then, a similarity search is executed using the attribute values of images obtained results (Jpn. Pat. Appln. KOKAI Publication No. 10-289240).
However, this search method also uses keywords. It is a heavy burden to provide images with keywords. Moreover, images having the same keyword may have markedly different image attribute values. Accordingly, a decrease in search accuracy cannot always be avoided even though a search for a similar image is executed on the basis of attribute values.